THE CIRCULATION. 173 



force of the oncoming venous blood and the contrary force of the 

 contracting auricle ; for the steady pressure of the blood in the cavee 

 has already been sufficient to open and command the auricle. The 

 latter then drives the blood into the ventricle, and so is relieved and 

 contracts ; and now the ventricle expands, and the blood which it 

 contains shuts upwards a triple valve, the tricuspid, which is at- 

 tached by tendinous cords to the muscular columns of the heart j 

 and the ventricle, reacting against its own forcible expansion, bear- 

 ing it no longer, throws out the blood, prevented from regurgitating 

 into the auricle by the tricuspid valve, into the pulmonary artery, 

 which is an artery carrying venous blood. The blood thus injected 

 into the pulmonary artery, is in its turn prevented from reflux by 

 three semilunar valves placed at its commencement, and it circulates 

 through another or second circle of arteries and veins, which con- 

 stitute the pulmonic circulation in contradistinction to the general 

 circulation upon which we have already expatiated. By the twigs 

 or ends of the pulmonary arteries it is returned into the twigs or 

 beginnings of the pulmonary veins ; and we may somewhat appreci- 

 ate the reason of an artery in this system carrying venous blood, and 

 a vein carrying arterial blood, when we learn, that this system is in an 

 important respect the inverse of the general system ; inasmuch as 

 the blood becomes de-arterialized, dark and venous in the capillary 

 vessels of the general circulation, but becomes re-arterialized and 

 crimson in the capillaries of the lungs or pulmonary system. 



It is not now necessary to consider the circulation of the lungs, 

 although it may be observed in passing, that the pulmonary arteries 

 and veins, running through those energetic air-pumps, can hardly 

 beat at any other times than during inspiration and expiration ; and 

 the pulse or stroke of the left ventricle can only act to the root of 

 the lungs, and fill the beginning of the pulmonary artery, as a reservoir 

 from which the lungs, at their own intervals, drink in the accumulated 

 blood. One function of the pericardium or heart-bag lies in eman- 

 cipating the heart from the power of the respiratory motions ; for 

 the heart lies in the midst of the lungs, grasped by their two arms, 

 the pulmonary arteries and veins, and were it naked or unprotected, 

 it would be drawn into the pulmonary votex, in which case its in- 

 voluntary life would cease, it would receive the immediate play of 



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